Process for synthesis of allophanate compounds and compositions including the product thereof

ABSTRACT

The invention provides a process for preparing an allophanate-containing compound in the presence of a metal catalyst and a tertiary amine. In a preferred process, the tertiary amine compound also has a hydroxyl group. The hydroxyl group can react with the isocyanate functionality. Also disclosed is a process in which an allophanate-containing compound is prepared with copper acetate monohydrate as a catalyst.  
     The allophanate-containing compound is used as a curing agent in coating compositions, particularly electrocoat coating compositions. The coating compositions are used to coat articles.

FIELD OF THE INVENTION

[0001] The invention concerns processes for preparing allophanatecompounds and thermosetting coating compositions that have allophanatecuring agents.

BACKGROUND OF THE INVENTION

[0002] Electrodeposition coating compositions and methods are widelyused in industry today. One of the advantages of electrocoatcompositions and processes is that the applied coating composition formsa uniform and contiguous layer over a variety of metallic substratesregardless of shape or configuration. This is especially advantageouswhen the coating is applied as an anticorrosive coating onto a substratehaving an irregular surface, such as a motor vehicle body. The even,continuous coating layer over all portions of the metallic substrateprovides maximum anticorrosion effectiveness.

[0003] Electrocoat baths usually comprise an aqueous dispersion of aprincipal film-forming resin, such as an acrylic or epoxy resin, havingionic stabilization. For automotive or industrial applications for whichhard electrocoat films are desired, the electrocoat compositions areformulated to be curable compositions. This is usually accomplished byincluding in the bath a crosslinking agent that can react withfunctional groups on the principal resin under appropriate conditions(such as with the application of heat) and thus cure the coating. Duringelectrodeposition, coating material containing an ionically-chargedresin having a relatively low molecular weight is deposited onto aconductive substrate by submerging the substrate in an electrocoat bathhaving dispersed therein the charged resin and then applying anelectrical potential between the substrate and a pole of oppositecharge, for example, a stainless steel electrode. The charged coatingmaterial migrates to and deposits on the conductive substrate. Thecoated substrate is then heated to cure the coating.

[0004] One curing mechanism utilizes a melamine formaldehyde resincuring agent in the electrodepositable coating composition to react withhydroxyl functional groups on the electrodeposited resin. This curingmethod provides good cure at relatively low temperatures (perhaps 130°C.), but the crosslink bonds contain undesirable ether linkages and theresulting coatings provide poor overall corrosion resistance as well aspoor chip and cyclic corrosion resistance.

[0005] In order to address some of the problems with melaminecross-linked electrocoats, many commercial compositions employpolyisocyanate crosslinkers to react with hydroxyl or amine functionalgroups on the electrodeposited resin. This curing method providesdesirable urethane or urea crosslink bonds, but it also entails severaldisadvantages. In order to prevent premature gelation of theelectrodepositable coating compositions, the highly reactive isocyanategroups on the curing agent must be blocked. Blocked polyisocyanates,however, require high temperatures, typically 175° C. or more to unblockand begin the curing reaction. In the past, the isocyanate crosslinkershave been blocked with a compound such as an oxime or alcohol, whichunblocks and volatilizes during cure, in order to provide the lowesttemperatures for the unblocking and curing reactions. The volatileblocking agents released during cure can cause other deleterious effectson various coating properties, however, and increase organic emissions.There is thus a need for electrodepositable coating compositions thatcould provide desirable urethane or urea crosslink linkages but thatavoid the problems that now accompany compositions having polyisocyanatecuring agents blocked with volatilizing agents.

SUMMARY OF THE INVENTION

[0006] I have now invented a process for making an allophanate compoundthat includes reacting a urethane group-containing precursor with anisocyanate group-containing compound in the presence of a metal catalystand a tertiary amine. The process of the invention offers an advantageof reduced time for the allophanate reaction, even for reduced levels ofmetal catalyst as compared to allophanate synthesis without the presenceof a tertiary amine. The present invention further provides a novelallophanate compound produced according the method just described.

[0007] In another embodiment, the invention offers a method of producinga compound containing at least one allophanate group which employscopper acetate monohydrate as catalyst. While metal acetylacetonatecatalysts have previously been used, copper acetate monohydrate offersadvantages in improved reaction and reduced cost of manufacture.

[0008] The invention further provides a coating composition, inparticular an electrocoat coating composition, that includes a compoundprepared according to the processes of the invention, in which thecompound has at least one allophanate group. Electrocoat coatingcompositions that include the allophanate-containing compound formed bythe process of the invention have unexpectedly improved throwpowerproperties. Electrocoat coating compositions that include the presentallophanate compound also have unexpectedly improved cure at lowertemperatures compared to current blocked-isocyanate electrocoatcompositions.

[0009] The present invention further furnishes a method of coating aconductive substrate. In the method of the invention, a conductivesubstrate is immersed in an electrodeposition coating compositioncomprising, in an aqueous medium, an ionic resin and a curing agenthaving at least one allophanate group; then, a potential of electriccurrent is applied between an electrode and the conductive substrate(which is then an electrode of the opposite charge) to deposit a coatinglayer onto the conductive substrate. The deposited coating layer iscured by is reaction between the ionic resin and the curing agent havingat least one allophanate group.

DETAILED DESCRIPTION OF THE INVENTION

[0010] The allophanate compounds of the invention are formed by reactingan excess of equivalents of organic polyisocyanate with a mono- orpolyhydric compound in the presence of a tertiary amine compound and acatalyst. The reaction is understood to involve formation of an initialurethane group which then, in the presence of the tertiary amine and thecatalyst, further reacts with an isocyanate to form the allophanategroup. In one embodiment of the invention, the tertiary amine compoundincludes one or more hydroxyl groups that may react with isocyanate toform a urethane group. The urethane group may then react with moreisocyanate to form allophanate functionality.

[0011] The equivalents of hydroxyl of mono- or polyhydric compoundemployed may range from about 0.01 to about 0.95 equivalents of hydroxylper equivalent of isocyanate. A more preferred range would be from about0.3 to about 0.75 equivalents of hydroxyl per equivalent of isocyanate,and even more preferred is from about 0.4 to about 0.6 equivalents ofhydroxyl per equivalent of isocyanate. In a preferred embodiment, thetertiary amine compound is an aminoalcohol. The aminoalcohol may beincluded in an amount of from about 0.01 to about 0.5 equivalentshydroxyl per equivalent of isocyanate. More preferably, the aminoalcoholis used in an amount of from about 0.01 to about 0.1 equivalentshydroxyl per equivalent isocyanate, and even more preferred is fromabout 0.01 to about 0.07 equivalents of hydroxyl per equivalent ofisocyanate. Although reaction conditions may be varied, the reaction maycontinue for 3 to 10 hours at temperatures of perhaps about 50° C. toabout 150° C. Progress of the reaction can be monitored by any of theusual methods, such as titration, infrared spectroscopy, or viscositymeasurement. A catalyst deactivator may optionally be added to stop theallophanate formation at a point where the desired isocyanate content orviscosity has been obtained. Addition of a deactivator is also desirablefor storage stability of the product with unreacted isocyanate content.Typically, the reaction may be allowed to continue to completion so thatthe product has substantially no residual isocyanate functionality.

[0012] Organic polyisocyanates that may be employed to prepare theallophanate containing compound include aromatic, aliphatic, andcycloaliphatic polyisocyanates and combinations thereof. Representativeof useful polyisocyanates are diisocyanates such as m-phenylenediisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate,mixtures of 2,4- and 2,6-toluene diisocyanate, hexamethylenediisocyanate, tetramethylene diisocyanate-cyclohexane-1,4-diisocyanate,any of the isomers of hexahydrotoluene diisocyanate, isophoronediisocyanate, any of the isomers of hydrogenated diphenylmethanediisocyanate, naphthalene-1,5-diisocyanate,1-methoxyphenyl-2,4-diisocyanate, any of the isomers of diphenylmethanediisocyanate, including 2,2′-diphenylmethane diisocyanate,2,4′-diphenylmethane diisocyanate, and 4,4′-diphenylmethanediisocyanate, isomers of biphenylene diisocyanate including 2,2′-,2,4′-, and 4.4′-biphenylene diisocyanates, 3,3′-dimethoxy-4,4′-biphenyldiisocyanate and 3,3′-dimethyl-diphenylmethane-4,4′-diisocyanate;triisocyanates such as 4,4′, 4″-triphenylmethane triisocyanate andtoluene 2,4,6-triisocyanate;

[0013] and the tetraisocyanates such as4,4′-dimethyldiphenylmethane-2,2′,5,5′-tetraisocyanate; and polymericpolyisocyanates such as polymethylene polyphenylene polyisocyanate.Especially useful due to their availability and properties are thevarious isomers of toluene diisocyanate and diphenylmethane diisocyanateand combinations of those isomers. Modified and oligomeric isocyanates,including isocyanurates, biurets, uretdione, and carbodiimidemodifications are also advantageously used to produce the allophanatecrosslinker. In one preferred embodiment, the polyisocyanate usedcomprises LUPERNATE® MI, LUPERNATE® MM103, both available from BASFCorp., Mt. Olive, N.J., or combinations thereof.

[0014] The mono- and polyhydric compounds that may be reacted with thepolyisocyanate may have an equivalent weight of about 30 to about 1000,can contain up to about 8 hydroxyl groups in the molecule, and can alsobe alkylene oxide adducts of lower molecular weight alcohols. Monohydricalcohols that may be employed include both aliphatic and aromaticalcohols. Suitable examples 10 include, without limitation, methanol,ethanol, propanol, 2-propanol, n-butanol, 2-chloroethanol, pentanol,n-octanol, 2-ethylhexanol, isooctyl alcohol, nonanol, ethylene glycolmonoalkyl ethers, propylene glycol monoalkyl ethers, diethylene glycolmonoalkyl ethers and higher molecular weight analogs of polyethyleneglycol monoalkyl ethers, dipropylene glycol monoalkyl ethers and highermolecular weight analogs of polypropylene glycol monoalkyl ethers,3,5,5-trimethylhexanol, isodecyl alcohol, benzyl alcohol, phenol,cyclohexanol, 2,2,2-tricholoroethanol, and the like, alkylene oxideadducts thereof, and combinations of these. The alkylene oxide may beethylene oxide, propylene oxide, butylene oxide, pentylene oxide, orcombinations thereof.

[0015] Suitable polyhydric compounds include both aliphatic and aromaticcompounds. Particular examples include, without limitation, ethyleneglycol, diethylene glycol, and higher polyethylene glycol analogs liketriethylene glycol; propylene glycol, dipropylene glycol, and higherpolypropylene glycol analogs like tripropylene glycol; 1,4-butanediol,1,3-butanediol, 1,6-hexanediol, 1,7-heptanediol, glycerine,1,1,1-trimethylolpropane, 1,1,1-trimethylolethane, hexane-1,2,6-triol,pentaerythritol, sorbitol, 4,4′-isopropylidene diphenol, (bisphenol A),resorcinol, catechol, hydroquinone, alkylene oxide adducts thereof andcombinations of these.

[0016] In a preferred embodiment, the process of the invention furtherutilizes a tertiary amine compound. The tertiary amine compoundcomprises at least one tertiary amine group, and may optionally compriseadditional tertiary amine groups. In a preferred embodiment, thetertiary amine compound comprises one or more alcohol groups. Suitableexamples of useful tertiary amine compounds include, without limitation,compounds having the structure

[0017] R¹N(R²)R³,

[0018] wherein R¹, R², and R³ are independently alkyl or hydroxyalkyl,preferably of from 1 to about 8 carbon atoms; or compounds having thestructure

[0019] R¹N(R²)—L—N(R⁴)R³,

[0020] wherein R¹, R², and R³ are as already defined, R⁴ is alkyl orhydroxyalkyl, preferably of from 1 to about 8 carbon atoms, and L is analkylene, arylene, or alkylarylene group, preferably having 1 to about15 carbon atoms; or substituted piperidines. Particular examples of suchcompounds include, without limitation, dimethylethanolamine,5-diethylamino-2-pentanol, 3-(diethylamino)-1,2-propanediol,3-diethylamino-1-propanol, 3-(dibenzylamino)-1-propanol,2-(dibutylamino)ethanol, 4-(dimethylamino)-1-methylpiperidine,4-(dimethylamino)phenethyl alcohol, 3-dimethylamino-2-propanol,2-(diisopropylamino)ethanol, 3-diisopropylamino-1 ,2-propanediol,triethanolamine, triethylamine, tributylamine, triisopropanolamine,triisodecylamine, triisobutylamine,N,N,N′,N′-tetraethyl-1,3-propanediamine,N,N,N′,N′-tetraethylethylenediamine,N,N,N′,N′-tetramethyl-1,6-hexanediamine, and so on, as well ascombinations of such compounds. When the allophanate compound is to beused as a curing agent in a cathodic electrocoat coating composition, itis preferred that the tertiary amine compound comprises at least onehydroxyl group so that the tertiary amine compound will react in thecuring agent synthesis and thus minimize the presence of low molecularweight amine compounds in the electrocoat bath. When the allophanatecompound is to be used as a curing agent in an anodic electrocoatcomposition, the tertiary amine compound may function as the saltingamine in the electrocoat coating composition.

[0021] When the allophanate compound of the invention is used in anelectrocoat composition, weight loss on curing can be further minimizedby selecting a low molecular weight tertiary amino alcohol whenpreparing the allophanate compound.

[0022] Useful catalysts for the allophanate synthesis are preferablythose transition metal compounds that are at least partially soluble inthe tertiary amine compound and that are known to promote formation ofallophanate linkages. Soluble zinc and copper compounds are preferred.Suitable examples of useful catalysts include, without limitation, metalcarboxylates, alcoholates, oxides, phenolates and metal chelates. In onepreferred embodiment, the catalyst is selected from acetylacetonates,including zinc, cobalt, nickel, ferric, copper, and aluminumacetylacetonates, and tin compounds, including dibutyltin dilaurate,dibutyltin oxide, stannous octoate, and dibutyltin diacetate. Whencombined with the tertiary amine compound, the catalyst may be includedin an unexpectedly reduced level as compared to the amount need to formallophanate compounds without the presence of the tertiary aminecompound. The reaction rate appears to be at least doubled as comparedto allophanate formation under the same conditions but without thetertiary amine compound.

[0023] In addition to known catalysts, it has been discovered thatcopper acetate monohydrate is an effective catalyst for the allophanateformation reaction. The copper acetate monohydrate offers a costadvantage over the metal acetoacetonate catatysts. The copper acetatemonohydrate also appears to be somewhat more effective in the reactionand can be used in lower amounts, based upon available copper (II)cation.

[0024] The catalyst is typically included in an amount of 0.0001 to0.001 equivalents per equivalent of isocyanate. Generally, the catalystmay be dissolved in the tertiary amine before being added to thereaction mixture.

[0025] The allophanate formation reaction may be continued until all ofthe isocyanate groups have reacted. In this case, when there is noresidual isocyanate functionality after the allophanate reaction, theallophanate compound may be used in the electrocoat coating compositionwithout further modification. In a different embodiment, however, theallophanate synthesis is ended while isocyanate functionality stillremains. The allophanate crosslinkers with residual isocyanatefunctionality may, optionally, be reacted in an additional reaction inwhich at least some of the residual isocyanate groups are reacted with ablocking agent and/or an isocyanate-reactive extender compound toproduce a blocked isocyanate and/or higher functionality crosslinker.The allophanate reaction may be stopped with residual isocyanatefunctionality, for example, to control the viscosity of theallophanate-functional product. It is preferred to have a viscosity,measured at 5020 C., of 500,000 cps or less, more preferably 100,000 cpsor less. In one particularly preferred embodiment, the allophanatereaction is continued until an isocyanate equivalent weight is obtainedthat is from about 200 to about 1200, more preferably from about 250 toabout 1000, and even more preferably from about 250 to about 400.

[0026] The reaction may effectively be stopped by reducing thetemperature, but it is often preferable to add a catalyst deactivator atthe desired point of the reaction. Examples of the catalyst deactivatorsthat may optionally be employed at the end of the reaction to preventfurther allophanate formation include, without limitation, aliphatic andaromatic acid chlorides such as acetyl chloride, benzoyl chloride,benzenesulfonyl chloride, oxalyl chloride, adipyl chloride, sebacylchloride, carbonyl chloride, and combinations of such compounds.Inorganic acid deactivators such as perchloric acid and strong organicacids such as trifluoromethanesulfonic acid and trifluoroacetic acid mayalso be used. Another group of catalyst deactivators that may be usedare chloroformates such as methyl chloroformate, ethyl chloroformate,isopropyl chloroformate, n-butyl chloroformate, sec-butyl chloroformate,and diethylene glycol bis chloroformate.

[0027] Optionally, the isocyanate-functional allophanate compound mayused in an additional reaction in which the residual isocyanate groupsare blocked and/or the compound is extended through reaction of theresidual isocyanate groups. Suitable blocking agent are those compoundsthat will unblock under the curing conditions to regenerate theisocyanate group for reaction as a crosslinking site. Blocking agentssuitable for crosslinkers for electrocoat coating compositions are knownin the art and include, without limitation, oximes, lower alcohols,lactams, and phenol. Specific examples of such materials include,without limitation, ethylene glycol monobutyl ether, diethylene glycolmonobutyl ether, methyl ethyl ketoxime, ε-caprolactam, and phenol.

[0028] Alternatively or in addition to reaction with a blocking agent,the isocyanate-functional allophanate precursor compound may be reactedwith an extender compound, which is an isocyanate reactive material thatis not expected to unblock and regenerate the isocyanate functionalityduring the curing reactions. Preferably, the extender compound is apolyfunctional compound that has two or more functional groups selectedfrom primary amine groups, secondary amine groups, and alcohol groups.The polyfunctional extender compounds act as extenders to link two ormore molecules of the allophanate precursor, producing a crosslinkerwith more allophanate groups per molecule. Useful examples of extendercompounds include aminoalcohols, polyfunctional amines, and polyols.Particular examples of such materials include, without limitation,trimethylolpropane, diethyl toluene diamine, trifunctional ordifunctional polyoxyalkylene amines (available commercially under thetradename POLYAMINE® from BASF Corporation or under the tradenameJEFFAMINE® from Huntsman), polyols such as those available under thetradenames PLURACOL® and PLURONIC® from BASF. The crosslinker preferablyhas no residual isocyanate functionality.

[0029] The crosslinker of the invention has at least about oneallophanate group per molecule on average and preferably has a pluralityof allophanate groups per molecule. The crosslinker preferably has up toabout 16, more preferably up to about 12, and even more preferably up toabout 8 allophanate groups per molecule on average. The crosslinker alsohas preferably more than about 1, more preferably at least about 2, andeven more preferably at least about 3 allophanate groups per molecule,on average. The crosslinker of the invention preferably has from about 1to about 16 allophanate groups on average per molecule, more preferablyhas from about 1 to about 12 allophanate groups on average per molecule,and even more preferably has from about 1 to about 8 allophanate groupson average per molecule. Typically, the crosslinker may have anequivalent weight of from about 200 to about 1200, based on combinedequivalents of allophanate and blocked isocyanate groups (if present).The weight average molecular weight may vary widely. In a preferredembodiment, the crosslinker of the invention has a weight averagemolecular weight of from about 2000 to about 15,000, more preferablyfrom about 4000 to about 12,000.

[0030] The electrocoat composition is an aqueous dispersion thatincludes at least a principal film-forming resin and the allophanatecuring agent of the invention. A variety of such resins are known,including without limitation, acrylic, polyester, epoxy, andpolybutadiene resins Preferably, the principal resin is cathodic, i.e.,it has basic groups and is salted with an acid. In a cathodicelectrocoating process, the article to be coated is the cathode.Water-dispersible resins used in the cathodic electrodeposition coatingprocess have a cationic functional group such as primary, secondary,tertiary, quarternary and/or amine moiety as a positively chargeablehydrophilic group.

[0031] In a preferred embodiment, the resin is an epoxy resinfunctionalized with amine groups. Preferably, the epoxy resin isprepared from a polyglycidyl ether. Preferably, the polyglycidyl etherof is the polyglycidyl ether of bisphenol A or similar polyphenols. Itmay also be advantageous to extend the epoxy resin by reacting an excessof epoxide group equivalents with a modifying material, such as apolyol, a polyamine or a polycarboxylic acid, in order to improve thefilm properties. Preferably, the polyglycidyl ether is extended withbisphenol A. Useful epoxy resins of this kind have a weight averagemolecular weight, which can be determined by GPC, of from about 3000 toabout 6000. Epoxy equivalent weights can range from about 200 to about2500, and are preferably from about 500 to about 1500.

[0032] Amino groups can be incorporated by reacting the polyglycidylethers of the polyphenols with amine or polyamines. Typical amines andpolyamines include, without limitation, dibutylamine, ethylenediamine,diethylenetriamine, triethylenetetramine, dimethylaminopropylamine,dimethylaminobutylamine, diethylaminopropylamine,diethylaminobutylamine, dipropylamine, and similar compounds, andcombinations thereof. In a preferred embodiment, the epoxide groups onthe epoxy resin are reacted with a compound comprising a secondary aminegroup and at least one latent primary amine. The latent primary aminegroup is preferably a ketimine group. After reaction with the epoxy theprimary amines are regenerated, resulting in an amine-capped epoxyresin. Resins used according to the invention preferably have a primaryamine equivalent weight of about 300 to about 3000, and more preferablyof about 850 to about 1300.

[0033] Epoxy-modified novolacs can be used as the resin in the presentinvention. The epoxy-novolac resin can be capped in the same way aspreviously described for the epoxy resin.

[0034] Acrylic polymers may be made cathodic by incorporation ofamino-containing monomers, such as acrylamide, methacrylamide, dimethylamino ethyl methacrylate or t-butyl amino ethyl methacrylate.Alternatively, epoxy groups may be incorporated by including anepoxy-functional monomer in the polymerization reaction. Suchepoxy-functional acrylic polymers may be made cathodic by reaction ofthe epoxy groups with polyamines according to the methods previouslydescribed for the epoxy resins. The molecular weight of a typicalacrylic resin is usually in the range from about 2000 to about 50,000,and preferably from about 3000 to about 15,000.

[0035] Cationic polyurethanes and polyesters may also be used. Suchmaterials may be prepared by endcapping with, for example, anaminoalcohol or, in the case of the polyurethane, the same compoundcomprising an amine group that can be salted as previously described mayalso be useful.

[0036] Polybutadiene, polyisoprene, or other epoxy-modified rubber-basedpolymers can be used as the resin in the present invention. Theepoxy-rubber can be capped with a compound comprising an amine group forsalting.

[0037] In an alternative embodiment, cationic or anionic acrylic resinsmay be used. In the case of a cationic acrylic resin, the resin ispolymerized using N,N′-dimethylaminoethyl methacrylate,tert-butylaminoethyl methacrylate, 2-vinylpyridine, 4-vinylpyridine,vinylpyrrolidine or other such amino monomers. In the case of an anionicacrylic resin, the resin is polymerized using acrylic acid, methacrylicacid, crotonic acid, maleic acid, fumaric acid, crotonic acid,isocrotonic acid, vinylacetic acid, and itaconic acid, anhydrides ofthese acids, or other suitable acid monomers or anhydride monomers thatwill generate an acid group for salting. The polymerization alsoincludes a hydroxyl-functional monomer. Useful hydroxyl-functionalethylenically unsaturated monomers include, without limitation,hydroxyethyl methacrylate, hydroxyethyl acrylate, hydroxypropylacrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate,hydroxybutyl methacrylate, the reaction product of methacrylic acid withstyrene oxide, and so on. Preferred hydroxyl monomers are methacrylic oracrylic acid esters in which the hydroxyl-bearing alcohol portion of thecompound is a linear or branched hydroxy alkyl moiety having from 1 toabout 8 carbon atoms. The monomer bearing the hydroxyl group and themonomer bearing the group for salting (amine for a cationic group oracid or anhydride for anionic group) may be polymerized with one or moreother ethylenically unsaturated monomers. Such monomers forcopolymerization are known in the art. Illustrative examples include,without limitation, alkyl esters of acrylic or methacrylic acid, e.g.,methyl methacrylate, ethyl acrylate, ethyl methacrylate, propylacrylate, propyl methacrylate, isopropyl acrylate, isopropylmethacrylate, butyl acrylate, butyl methacrylate, isobutyl acrylate,isobutyl methacrylate, t-butyl acrylate, t-butyl methacrylate, amylacrylate, amyl methacrylate, isoamyl acrylate, isoamyl methacrylate,hexyl acrylate, hexyl methacrylate, 2-ethylhexyl acrylate, decylacrylate, decyl methacrylate, isodecyl acrylate, isodecyl methacrylate,dodecyl acrylate, dodecyl methacrylate, cyclohexyl acrylate, cyclohexylmethacrylate, substituted cyclohexyl acrylates and methacrylates,3,5,5-trimethylhexyl acrylate, 3,5,5-trimethylhexyl methacrylate, thecorresponding esters of maleic, fumaric, crotonic, isocrotonic,vinylacetic, and itaconic acids, and the like; and vinyl monomers suchas styrene, t-butyl styrene, alpha-methyl styrene, vinyl toluene and thelike. Other useful polymerizable co-monomers include, for example,alkoxyethyl acrylates and methacrylates, acryloxy acrylates andmethacrylates, and compounds such as acrylonitrile, methacrylonitrile,acrolein, and methacrolein. Combinations of these are usually employed.

[0038] The amino equivalent weight of the cationic resin can range fromabout 150 to about 5000, and preferably from about 500 to about 2000.The hydroxyl equivalent weight of the resins, if they have hydroxylgroups, is generally between about 150 and about 2000, and preferablyabout 200 to about 800.

[0039] The electrodeposition coating composition may further containconventional pigments such as titanium dioxide, ferric oxide, carbonblack, aluminum silicate, precipitated barium sulfate, aluminumphosphomolybdate, strontium chromate, basic lead silicate or leadchromate. The pigments may be dispersed using a grind resin or,preferably, a pigment dispersant. The pigment- to-resin weight ratio inthe electrocoat bath can be important and should be preferably less than50:100, more preferably less than 40:100, and usually about 10 to30:100. Higher pigment-to-resin solids weight ratios have been found toadversely affect coalescence and flow. Usually, the pigment is 10-40percent by weight of the nonvolatile material in the bath. Preferably,the pigment is 15 to 30 percent by weight of the nonvolatile material inthe bath. Any of the pigments and fillers generally used in electrocoatprimers may be included. Extenders such as clay and anti-corrosionpigments are commonly included.

[0040] The above components are uniformly dispersed in an aqueousmedium. Usually, the principal resin and the crosslinking agent areblended together before the resins are dispersed in the water. In apreferred embodiment, the amine groups of the cathodic electrocoatresins are salted with an acid, such as phosphoric acid, propionic acid,acetic acid, lactic acid, or citric acid. The salting acid may beblended with the resins, mixed with the water, or both, before theresins are added to the water. The acid is used in an amount sufficientto neutralize enough of the amine groups of the principal resin toimpart water-dispersibility to the resin. The cationic resin may befully neutralized; however, partial neutralization is usually sufficientto impart the required water-dispersibility. By “partial neutralization”we mean that at least one, but less than all, of the basic groups on theresin are neutralized. By saying that the cationic resin is at leastpartially neutralized, we mean that at least one of the basic groups onthe resin is neutralized, and up to all of such groups may beneutralized. The degree of neutralization that is required to afford therequisite water-dispersibility for a particular resin will depend uponits chemical composition, molecular weight, and other such factors andcan readily be determined by one of ordinary skill in the art throughstraightforward experimentation.

[0041] Similarly, the acid groups of an anionic resin are salted with anamine such as dimethylethanolamine or triethylamine. Again, the saltingagent-(in this case, an amine) may be blended with the resins, mixedwith the water, or both, before the resins are added to the water. Theanionic principal resin is at least partially neutralized, but may befully neutralized as in the case of the cationic resin. At least enoughacid groups are salted with the amine to impart water-dispersibility tothe resin.

[0042] Besides water, the aqueous medium of an electrocoat compositionmay also contain a coalescing solvent. Useful coalescing solventsinclude hydrocarbons, alcohols, esters, ethers and ketones. Thepreferred coalescing solvents include alcohols, polyols and ketones.Specific coalescing solvents include monobutyl and monohexyl ethers ofethylene glycol, and phenyl ether of propylene glycol, monoalkyl ethersof ethylene glycol such as the monomethyl, monoethyl, monopropyl, andmonobutyl ethers of ethylene glycol; dialkyl ethers of ethylene glycolsuch as ethylene glycol dimethyl ether; or diacetone alcohol. A smallamount of a water-immiscible organic solvent such as xylene, toluene,methyl isobutyl ketone or 2-ethylhexanol may be added to the mixture ofwater and the water-miscible organic solvent. The amount of coalescingsolvent is not critical and is generally between about 0 to 15 percentby weight, preferably about 0.5 to 5 percent by weight based on totalweight of the resin solids.

[0043] The electrodeposition coating compositions used in the inventioncan contain optional ingredients such as dyes, flow control agents,plasticizers, catalysts, wetting agents, surfactants, UV absorbers, HALScompounds, antioxidants, defoamers and so forth. Examples of surfactantsand wetting agents include alkyl imidazolines such as those availablefrom Ciba-Geigy Industrial Chemicals as AMINE C® acetylenic alcoholssuch as those available from Air Products and Chemicals under thetradename SURFYNOL®. Surfactants and wetting agents, when present,typically amount to up to 2 percent by weight resin solids. Plasticizersare optionally included to promote flow or modify plating properties.Examples are high boiling water immiscible materials such as ethylene orpropylene oxide adducts of nonyl phenols or bisphenol A. Plasticizerscan be used at levels of up to 15 percent by weight resin solids.

[0044] Curing catalysts such as tin catalysts can be used in the coatingcomposition. Typical examples are without limitation, tin and bismuthcompounds including dibutyltin dilaurate, dibutyltin oxide, and bismuthoctoate. When used, catalysts are typically present in amounts of about0.05 to 2 percent by weight tin based on weight of total resin solids.

[0045] The electrocoat bath generally has an electroconductivity from800 micromhos to 6000 micromhos. When conductivity is too low, it isdifficult to obtain a film of desired thickness and having desiredproperties. On the other hand, if the composition is too conductive,problems such as the dissolution of substrate or counter electrode inthe bath, uneven film thickness, rupturing of the film, or poorresistance of the film to corrosion or water spotting may result.

[0046] The coating composition according to the present invention iselectrodeposited onto a substrate and then cured to form a coatedarticle. The electrodeposition of the coating preparations according tothe invention may be carried out by any of a number of processes knownto those skilled in the art. The electrodeposition coating compositionmay be applied on any conductive substrate, such as steel, copper,aluminum, or other metals or metal alloys, preferably to a dry filmthickness of 10 to 35 μm. The article coated with the composition of theinvention may be a metallic automotive part or body. After application,the coated article is removed from the bath and rinsed with deionizedwater. The coating may be cured under appropriate conditions, forexample by baking at from about 275° F. to about 375° F. for betweenabout 15 and about 60 minutes.

[0047] Following electrodeposition, the applied coating is usually curedbefore other coatings, if used, are applied. When the electrocoat layeris used as a primer in automotive applications, one or more additionalcoating layers, such as a primer-surfacer, color coat, and, optionally,a clearcoat layer, may be applied over the electrocoat layer The colorcoat may be a topcoat enamel. In the automotive industry, the color coatis often a basecoat that is overcoated with a clearcoat layer. Theprimer surfacer and the topcoat enamel or basecoat and clearcoatcomposite topcoat may be ether waterborne or solventborne. The coatingscan be formulated and applied in a number of different ways known in theart. For example, the resin used can be an acrylic, a polyurethane, or apolyester. Typical topcoat formulations are described in U.S. Pat. Nos.4,791,168, 4,414,357, 4,546,046, 5,373,069, and 5,474,811. The coatingscan be cured by any of the known mechanisms and curing agents, such as amelamine or blocked isocyanate.

[0048] The invention is further described in the following example. Theexample is merely illustrative and does not in any way limit the scopeof the invention as described and claimed. All parts are parts by weightunless otherwise noted.

EXAMPLE 1. Preparation of Crosslinker having Allophanate Groups

[0049] A suitable reactor was charged with 647.6 grams of LUPRANATEMM103 (available from BASF Corporation) and 266.4 grams of LUPRANATE MI(available from BASF Corporation) under a blanket of nitrogen. Themixture was heat to about 45° C., at which time 338.3 grams of ethyleneglycol monobutyl ether was added over a period of about 30 minutes. Thetemperature rose to 60° C. After 30 minutes, 300.0 grams of methylisobutyl ketone was added, followed by a solution of 0.928 grams ofcopper acetate monohydrate in 41.49 grams of dimethylethanolamine.Another 24.0 grams of methyl isobutyl ketone was added to rinse theaddition funnel. The mixture was heated to 75° C. and then allowed toexotherm to a maximum temperature of 88° C. After about 40 minutes, withthe reaction mixture being held at 75° C., about 406 grams of methylisobutyl ketone were added slowly. The reaction mixture was held at 75°C. until all of the isocyanate functionality was consumed, based oninfrared spectroscopy. The resulting product has a nonvolatile contentof 65.0% by weight.

EXAMPLE 2. Preparation of Electrocoat Coating Composition

[0050] An electrocoat emulsion was prepared according to the followingmethod. In a suitable container, 658.1 grams of an epoxy solution(greater than 778 weight per epoxide) is cooled from a reactiontemperature of 133° C. to a temperature of 100° C. for addition 116.0grams of a plasticizer mixture (62% nonvolatiles). At 85° C., 35.0 gramsof the diketimine of diethylene triamine, 38.8 grams ofmethylethanolamine, and 10.0 grams of propylene glycol phenyl ether wereadded. After 35 minutes, the reaction mixture was cooled to 103° C. andreduced to about 79% nonvolatile by weight with 97.1 grams of a mixtureof isobutanol and additives. The reaction mixture was then cooled to 91°C. for the addition of 678.0 grams of the crosslinker from Example 1, toproduce the final resin mixture at about 74% nonvolatile by weight.

[0051] A two-gallon vessel was charged with 747.0 grams of deionizedwater and 51.6 grams of 88% lactic acid. An amount of about 1590 gramsof the final resin mixture was added with good mixing. A total of 1600additional grams of deionized water were added in portions with goodmixing to produce an emulsion with a nonvolatile content of 30% byweight. Organic solvent was stripped from the emulsion and additionaldeionized water added. The final emulsion had a nonvolatile content of32.1% by weight. The extent of neutralization was 48%.

[0052] The emulsion was then used to prepare an electrocoat coatingcomposition (electrocoat bath). In a separate container, 1633 grams ofthe final emulsion, 281 grams of a pigment paste (63% by weightnonvolatile, pigment-to-binder=3.3), and 1986 grams of deionized waterwere mixed together. The electrocoat bath was mixed for 2 hours in anopen vessel. The bath had a nonvolatile content of 18% by weight, and pHof 5.8, and a conductivity of 1600 micromhos.

EXAMPLE 3. Preparation of Crosslinker having Allophanate Groups

[0053] A crosslinker having allophanate groups was prepared as describedin Example 1, but the copper acetate monohydrate was replaced on anequivalent basis with zinc acetylacetonate monohydrate.

EXAMPLE 4. Preparation of Electrocoat Coating Composition

[0054] An electrocoat coating composition was prepared according toExample 2, but using the crosslinker of Example 3 in place of thecrosslinker of Example 1.

[0055] Evaluation of Coating Compositions

[0056] The electrocoat coating compositions of Examples 2 and 4 wereused to coat 4″×12″ steel panels. Panels were coated at 90° F. for 2.2minutes at 250 volts. The deposited coatings were cured by baking for 20minutes at 350° F.

[0057] The panels coated from the example compositions were evaluatedand compared to results using a commercial product, Cathoguard® 310G,available from BASF Corp., having a standard blocked isocyanatecrosslinker. The percent weight losses are given in the following table.Comparative Example 2 Example 4 Example Throwpower, mm 279 279 203Protective 278 279 190 throwpower, mm¹ 500 Hour salt 1.40 1.49 1.23spray (mm scribe creep) GM9540P 2.99 2.57 2.59 (General Motors 40 cyclecorrosion test) (mm scribe creep)

[0058] The invention has been described in detail with reference topreferred embodiments thereof. It should be understood, however, thatvariations and modifications can be made within the spirit and scope ofthe invention and of the following claims.

What is claimed is:
 1. A process for preparing an allophanate-containing compound, comprising a step of reacting a compound having urethane functionality with a compound having isocyanate functionality in the presence of a metal catalyst and a tertiary amine.
 2. A process according to claim 1 , wherein the reaction between the compound having urethane functionality and the compound having isocyanate functionality is produced by reacting an excess of equivalent of a polyisocyanate compound with a hydroxyl-functional compound.
 3. A process according to claim 2 , wherein the polyisocyanate is at least one member selected from the group consisting of isomers of diphenyl methane diisocyanate and mixtures thereof, isocyanurates, biurets, uretdione-containing compounds, carbodiimide-containing compounds, and combinations of these.
 4. A process according to claim 2 , wherein the hydroxyl-functional compound is at least one member selected from the group consisting of n-butanol, 2-chloroethanol, 2-ethylhexanol, ethylene glycol monoalkyl ethers, propylene glycol monoalkyl ethers, benzyl alcohol, phenol, ethylene glycol polyethylene glycols, propylene glycol, polypropylene glycols, butanediols, trimethylolpropane, pentaerythritol, and alkylene oxide adducts thereof, and combinations thereof.
 5. A process for preparing an allophanate-containing compound, comprising a step of reacting: (a) a compound having at least one urethane group; (b) a compound having at least one isocyanate group; (c) a compound comprising at least one hydroxyl group and at least one tertiary amine group; and (d) a metal catalyst.
 6. A process according to claim 5 , wherein compound (c) is included in amount of from about 0.01 to about 0.1 equivalents hydroxyl per equivalent of isocyanate.
 7. A process according to claim 5 , wherein compound (c) is selected from the group consisting of dimethylethanolamine, diethylaminopentanol, diethylaminopropanol, dimethylaminopropanol, triethanolamine, and combinations thereof.
 8. A process according to claim 5 , wherein the metal catalyst is copper acetate monohydrate.
 9. An allophanate-containing compound prepared according to the process of claim 5 .
 10. A process for preparing an allophanate-containing compound, comprising a step of reacting a compound having urethane functionality with a compound having isocyanate functionality in the presence of copper acetate monohydrate.
 11. A process according to claim 10 , wherein the reaction between the compound having urethane functionality and the compound having isocyanate functionality is produced by reacting an excess of equivalent of a polyisocyanate compound with a hydroxyl-functional compound.
 12. A process according to claim 1 1, wherein the polyisocyanate is at least one member selected from the group consisting of isomers of diphenyl methane diisocyanate and mixtures thereof, isocyanurates, biurets, uretdione-containing compounds, carbodiimide-containing compounds, and combinations of these.
 13. A process according to claim 11 , wherein the hydroxyl-functional compound is at least one member selected from the group consisting of n-butanol, 2-chloroethanol, 2-ethylhexanol, ethylene glycol monoalkyl ethers, propylene glycol monoalkyl ethers, benzyl alcohol, phenol, ethylene glycol polyethylene glycols, propylene glycol, polypropylene glycols, butanediols, trimethylolpropane, pentaerythritol, and alkylene oxide adducts thereof, and combinations thereof.
 14. A coating composition comprising an allophanate-containing compound and a resin reactive with the allophanate-containing compound, wherein the allophanate-containing compound is prepared by a process comprising a step of reacting a compound having urethane functionality with a compound having isocyanate functionality in the presence of a metal catalyst and a tertiary amine.
 15. A coating composition according to claim 14 , wherein the tertiary amine comprises a hydroxyl group.
 16. An electrocoat coating composition comprising, in an aqueous medium, (a) an ionic resin having functionality reactive with isocyanate groups and (b) a compound comprising at least one allophanate group that is prepared by a process comprising a step of reacting a mixture comprising a compound having urethane functionality, a compound having isocyanate functionality, and a compound comprising a hydroxyl group and a tertiary amine group.
 17. An electrocoat coating composition according to claim 16 , wherein the process for preparing the compound (b) includes a further step of reacting an allophanate-containing compound having isocyanate functionality with an isocyanate-reactive compound selected from the group consisting of blocking agents, extender compounds, and combinations thereof.
 18. An electrocoat coating composition according to claim 16 , wherein, in which the process for preparing the compound (b), the step of reacting a mixture comprising a compound having urethane functionality, a compound having isocyanate functionality, and a compound comprising a hydroxyl group and a tertiary amine group is continued until there is substantially no residual isocyanate functionality.
 19. An electrocoat coating composition according to claim 17 , wherein the isocyanate-reactive compound is selected from the group consisting of oximes, lactams, phenol, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, and combinations thereof.
 20. An electrocoat coating composition according to claim 17 , wherein the isocyanate-reactive compound comprises a compound that has two or more functional groups selected from the group consisting of primary amine groups, secondary amine groups, alcohol groups, and combinations thereof.
 21. An electrocoat coating composition according to claim 20 , wherein the compound that has two or more functional groups is selected from the group consisting of trimethylolpropane, diethyl toluene diamine, trifunctional polyoxyalkylene amines, difunctional polyoxyalkylene amines, and combinations thereof.
 22. An electrocoat coating composition according to claim 16 , wherein the compound (b) has an equivalent weight of from about 200 to about 1200, based on combined equivalents of allophanate and blocked isocyanate groups.
 23. An electrocoat coating composition according to claim 16 , wherein the compound (b) has up to about 8 allophanate groups.
 24. An electrocoat coating composition according to claim 16 , wherein the resin (a) is cationic.
 25. An electrocoat coating composition according to claim 24 , wherein the resin (a) is an epoxy resin.
 26. An electrocoat coating composition according to claim 16 , wherein the resin (a) is anionic.
 27. A method of coating a conductive substrate, comprising the steps of: (a) providing an aqueous coating composition comprising an ionic resin having functionality reactive with isocyanate and a compound comprising at least one allophanate group, wherein the compound comprising at least one allophanate group is prepared by a process comprising a step of reacting a mixture comprising a compound having urethane functionality, a compound having isocyanate functionality, and a compound comprising a hydroxyl group and a tertiary amine group; (b) immersing a conductive substrate in said electrodeposition coating composition; (c) applying a potential of electric current between an electrode and the conductive substrate to deposit a coating layer onto the conductive substrate; and (d) curing the deposited coating layer by reacting of the resin having functionality reactive with isocyanate and the compound comprising at least one allophanate group.
 28. A method according to claim 27 , wherein the ionic resin is cationic.
 29. A method according to claim 28 , wherein the cationic resin is an epoxy resin.
 30. A method according to claim 27 , wherein the ionic resin is anionic.
 31. An article coated according to the process of claim 27 . 